Mr Moon narrowly lost to Ms Park in the last presidential election in 2012.

He has been critical of the two previous conservative administrations for failing to stop North Korea’s weapons development.

A North Korean endorsement: Stephen Evans, BBC News, Seoul

North Korea has dropped a pretty clear hint about who it wants to win the election. A state media editorial says the current North and South relationship has been “been wrought by the conservative groups which, having been in power for the past 10 years, revived the foregone period of confrontation and maximized the political and military rivalry between the same race”.

This indicates it favours a candidate associated with what’s known as the Sunshine Policy when Pyongyang and Seoul communicated and co-operated.

The policy finally collapsed in 2008 after North Korea’s started testing nuclear devices, though it had faltered earlier when South Korea stopped sending aid to the north in 2006.

Moon Jae-in was part of the government which promoted the Sunshine Policy. He has indicated he wants closer ties with the North in this campaign. An endorsement by Pyongyang won’t help him win – but it probably won’t harm his chances very much either.

Observers are expecting a high turnout at the ballot boxes, with numbers boosted by younger voters.

Despite the constant turbulence with North Korea, domestic issues are foremost in the minds of most voters.

All the candidates are promising to protect the fragile recovery in South Korea’s economy – the fourth largest in Asia – and to bring down youth unemployment, which remains stubbornly high.

There are also promises to rein in perceptions of elitism in South Korean political society and reform the family-run conglomerates – chaebols – which dominate the domestic economy.

Polls close at 20:00 local time (11:00 GMT), with the winner expected to be announced soon after.